<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>Document</title>

    <style>
      .box {
        width: 200px;
        height: 200px;
        border: 10px solid skyblue;
        padding: 10px;
        margin: 100px auto;
        /* 
            添加 box-sizing 之前，box是外扩的，总宽度=240px
            添加 box-sizing 之后，变成内缩，总宽度=200px */
        box-sizing: border-box;
        /* button 按钮默认是 怪异盒模型 */
      }
    </style>
  </head>
  <body>
    <body>
      <div class="box"></div>
    </body>
  </body>
</html>
